Field
The present invention relates to an acoustic transducer and a microphone. Specifically, the present invention relates to a capacitance type of acoustic transducer configured by a capacitor structure made up of a vibrating electrode plate (diaphragm) and a fixed electrode plate. The present invention also relates to a microphone that employs this acoustic transducer. In particular, the present invention relates to a very small-sized acoustic transducer created using MEMS (Micro Electro Mechanical System) technology.
Related Art
In recent years, there has been demand for microphones to detect sounds with high sensitivity in a range from low sound pressure to high sound pressure. In general, the maximum input sound pressure of a microphone is limited by the harmonic distortion rate (total harmonic distortion). This is because when a microphone attempts to detect a sound having a high sound pressure, harmonic distortion occurs in the output signal, and the sound quality and precision become impaired. Accordingly, if the harmonic distortion rate can be reduced, it is possible to raise the maximum input sound pressure and widen the detectable sound pressure range (referred to hereinafter as the “dynamic range”) of the microphone.
However, in general microphones, there is a trade-off relationship between an improvement in the acoustic vibration detection sensitivity and a reduction in the harmonic distortion rate, and it has been difficult to provide a microphone with a wide dynamic range from low-volume (low sound pressure) sounds to high-volume (high sound pressure) sounds.
In this technical background, a method of using of an acoustic sensor structured as shown in FIGS. 1A and 1B has been proposed as a method for realizing a microphone that has a wide dynamic range. FIG. 1A is a cross-sectional diagram of an acoustic sensor 11 according to a conventional example, and FIG. 1B is a plan view of a state where a back plate 19 has been removed.
In the acoustic sensor 11, a first diaphragm 16a and a second diaphragm 16b that are divided by a slit 17 are arranged above a substrate 12 that has a cavity 13. The first diaphragm 16a has a relatively larger area and is supported on the upper surface of the substrate 12 by anchors 18a. The second diaphragm 16b has a relatively smaller area and is supported on the upper surface of the substrate 12 by anchors 18b. A back plate 19 is provided on the upper surface of the substrate 12 so as to cover the two diaphragms 16a and 16b, and a first fixed electrode plate 20a and a second fixed electrode plate 20b are arranged on the lower surface of the back plate 19 so as to oppose the first diaphragm 16a and the second diaphragm 16b. A large number of acoustic holes 21 are formed in the back plate 19 and the fixed electrode plates 20a and 20b. 
In the acoustic sensor 11, a high-sensitivity first acoustic sensing portion 14 that can detect low-volume (low sound pressure) sounds is configured by the first diaphragm 16a and the first fixed electrode plate 20a that oppose each other. Also, a low-sensitivity second acoustic sensing portion 15 that can detect high-volume (high sound pressure) sounds is configured by the second diaphragm 16b and the second fixed electrode plate 20b that oppose each other. Also, the output from the acoustic sensor 11 is switched between output from the first acoustic sensing portion 14 and output from the second acoustic sensing portion 15 according to the volume, thus making it possible to detect sounds with high sensitivity in a range from low sound pressure to high sound pressure. One example of such an acoustic sensor is disclosed in JP 2012-147115A.
JP 2012-147115A is an example of background art.